Current-perpendicular-to-the-plane (cpp) magnetoresistive sensor with antiparallel-free layer structure and low current-induced noise

ABSTRACT

A current-perpendicular-to-the-plane (CPP) magnetoresistive sensor has an antiparallel free (APF) structure as the free layer and a specific direction for the applied bias or sense current. The (APF) structure has a first free ferromagnetic (FL 1 ), a second free ferromagnetic layer (FL 2 ), and an antiparallel (AP) coupling (APC) layer that couples FL 1  and FL 2  together antiferromagnetically with the result that FL 1  and FL 2  have substantially antiparallel magnetization directions and rotate together in the presence of a magnetic field. The thickness of FL 1  is preferably greater than the spin-diffusion length of the electrons in the FL 1  material. The minimum thickness for FL 2  is a thickness resulting in a FL 2  magnetic moment equivalent to at least 10 Å Ni 80 Fe 20  and preferably to at least 15 Å Ni 80 Fe 20 . The CPP sensor operates specifically with the conventional sense current (opposite the electron current) directed from the pinned ferromagnetic layer to the APF structure, which results in suppression of current-induced noise.

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.11/380,625 filed Apr. 27, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a current-perpendicular-to-the-plane(CPP) magnetoresistive sensor that operates with the sense currentdirected perpendicularly to the planes of the layers making up thesensor stack, and more particularly to a CPP sensor with lowcurrent-induced noise.

2. Background of the Invention

One type of conventional magnetoresistive sensor used as the read headin magnetic recording disk drives is a “spin-valve” (SV) sensor. A SVmagnetoresistive sensor has a stack of layers that includes twoferromagnetic layers separated by a nonmagnetic electrically conductivespacer layer, which is typically copper (Cu). One ferromagnetic layerhas its magnetization direction fixed, such as by being pinned byexchange coupling with an adjacent antiferromagnetic layer, and theother ferromagnetic layer has its magnetization direction “free” torotate in the presence of an external magnetic field. With a sensecurrent applied to the sensor, the rotation of the free-layermagnetization relative to the fixed-layer magnetization is detectable asa change in electrical resistance.

In a magnetic recording disk drive SV read sensor or head, the stack oflayers are located in the read “gap” between magnetic shields. Themagnetization of the fixed or pinned layer is generally perpendicular tothe plane of the disk, and the magnetization of the free layer isgenerally parallel to the plane of the disk in the absence of anexternal magnetic field. When exposed to an external magnetic field fromthe recorded data on the disk, the free-layer magnetization will rotate,causing a change in electrical resistance. If the sense current flowingthrough the SV is directed parallel to the planes of the layers in thesensor stack, the sensor is referred to as a current-in-the-plane (CIP)sensor, while if the sense current is directed perpendicular to theplanes of the layers in the sensor stack, it is referred to ascurrent-perpendicular-to-the-plane (CPP) sensor.

CPP-SV read heads are described by A. Tanaka et al., “Spin-valve headsin the current-perpendicular-to-plane mode for ultrahigh-densityrecording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January2002. Another type of CPP sensor is a magnetic tunnel junction (MTJ)sensor in which the nonmagnetic spacer layer is a very thin nonmagnetictunnel barrier layer. In a MTJ magnetoresistive read head the spacerlayer is electrically insulating and is typically alumina (Al₂O₃ ) orMgO; in a CPP-SV magnetoresistive read head the spacer layer iselectrically conductive and is typically copper.

CPP sensors are susceptible to current-induced noise and instability.The spin-polarized current flows perpendicularly through theferromagnetic layers and produces a spin transfer torque on the localmagnetization. This can produce continuous gyrations of themagnetization, resulting in substantial low-frequency magnetic noise ifthe sense current is above a certain level. This effect is described byJ.-G. Zhu et al., “Spin transfer induced noise in CPP read heads,” IEEETRANSACTIONS ON MAGNETICS, Vol. 40, pp. 182-188, January 2004. In arelated paper it was suggested, but not demonstrated, that thesensitivity to spin-torque-induced instability of the free layer couldbe reduced by use of a dual spin-valve. (J.-G. Zhu et al., “Currentinduced noise in CPP spin valves,” IEEE TRANSACTIONS ON MAGNETICS, Vol.40, No. 4, pp. 2323-2325, July 2004). However, a dual spin-valverequires a second spacer layer on the free layer and a second pinnedlayer on the second spacer layer, and thus results in a largershield-to-shield read gap distance, which lowers sensor resolution.

What is needed is a CPP sensor that produces minimal current-inducednoise without loss of magnetoresistance or sensor resolution.

SUMMARY OF THE INVENTION

The invention is a CPP magnetoresistive sensor with an antiparallel free(APF) structure and a specific direction for the applied bias or sensecurrent that result in both increased magnetoresistance and, moreimportantly, reduced susceptibility to current-induced instability andnoise. The (APF) structure has a first free ferromagnetic (FL1) adjacentthe CPP sensor's nonmagnetic spacer layer, a second free ferromagneticlayer (FL2), and an antiparallel (AP) coupling (APC) layer that couplesFL1 and FL2 together antiferromagnetically with the result that FL1 andFL2 have substantially antiparallel magnetization directions. The sensormay also include a ferromagnetic biasing layer formed from CoPt or someother hard magnetic material outside of, and electrically insulatedfrom, the sensor stack near the side edges of the free ferromagneticlayer (typically FL1), with sufficiently large magnetic moment tolongitudinally bias the net magnetization of the APF structure. Theantiferromagnetically-coupled FL1 and FL2 rotate together in thepresence of a magnetic field, such as the magnetic fields from datarecorded in a magnetic recording medium. The thicknesses of FL1 and FL2are chosen to obtain the desired net free layer magnetic moment/area forthe sensor, which for advanced CPP read heads is no greater than theequivalent moment/area of approximately 100 Å of Ni₈₀Fe₂₀. The sensor isbased on the discovery that for a CPP sensor with an appropriate APFstructure, the critical current above which current-induced noise occursis substantially higher when the applied sense current is directed fromthe pinned layer to the APF structure. Thus the sensor operates with thesense current I_(S) (which is the conventional current and opposite indirection to the electron current I_(e)) directed from the referenceferromagnetic layer to the APF structure, so that the spin-torqueeffect, and thus current-induced noise, is suppressed. This allows muchhigher sense currents to be used, resulting in higher sensor outputvoltage.

Also, to maximize the sensor signal, the thickness of FL1 can be chosento be greater than the spin-diffusion length of electrons in the FL1material, which maximizes the bulk spin-dependent scattering in FL1. Itis also known, through both direct experimentation and modeling, thatthe critical current that results in free-layer instability can besubstantially increased with increasing thickness of FL2, so that itwould appear to be desirable to maximize the thickness of FL2 oroptimize it relative to the thickness of FL1. However, because themagnetization of FL2 is antiparallel to the magnetization of the biasinglayer in the absence of an external magnetic field, if the magneticmoment of FL2 is too high, which can occur if FL2 is too thick, the APFstructure may become magnetostatically unstable (at any bias current).

In this invention it has also been discovered, through experiment andmodeling, that an FL2 thickness resulting in a magnetic momentequivalent to about 15 Å Ni₈₀Fe₂₀ gives a substantial increase incritical current and that the absolute value of the critical current isrelatively independent of FL1 thickness when FL2 is fixed at thisequivalent thickness. Thus, although the free-layer critical current forthe APF structure may be increased even further with thicker FL2, such athicker FL2 will likely offer little to no spin-torque stabilityadvantage for the read head, which will instead likely be limitedinstead by the reference layer critical current. Rather a thicker FL2will quickly weaken magnetostatic stability. Thus, keeping FL2 nothicker than that needed to provide enough margin in critical current sothat spin-torque-induced free-layer instability is not the limitingfactor on read head performance, will also minimize magnetostaticinstability of the FL1/FL2 couple.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic top view of a conventional magnetic recording harddisk drive with the cover removed.

FIG. 2 is an enlarged end view of the slider and a section of the disktaken in the direction 2-2 in FIG. 1.

FIG. 3 is a view in the direction 3-3 of FIG. 2 and shows the ends ofthe read/write head as viewed from the disk.

FIG. 4 is a cross-sectional schematic view of acurrent-perpendicular-to-the-plane spin-valve (CPP-SV) read head showingthe stack of layers located between the magnetic shield layers.

FIG. 5 is a cross-sectional schematic view of a CPP-SV read headaccording to this invention.

FIG. 6 is a graph of noise power spectral density (PSD) as a function ofelectron current I_(e) at positive and negative applied magnetic fieldsfor a control sample according to the prior art with a single freelayer, wherein positive electron current I_(e) corresponds to electronflow from the pinned layers towards the free layers.

FIG. 7 is a graph of noise power spectral density (PSD) as a function ofelectron current I_(e) at positive and negative applied magnetic fieldsfor a sample according to this invention with an antiparallel-freestructure, wherein positive electron current I_(e) corresponds toelectron flow from the pinned layers towards the free layers.

FIG. 8 is a graph of positive and negative critical electron currentsI_(e) ^(crit) for spin-torque-induced noise as a function of the secondfree ferromagnetic layer (FL2) thickness.

FIG. 9 is a numerical simulation of the negative critical currentdensity (for the parallel state) for an antiparallel free (APF)structure as a function of the first free ferromagnetic layer (FL1)thickness for four values of FL2 thickness.

DETAILED DESCRIPTION OF THE INVENTION

The CPP-SV read head has application for use in a magnetic recordingdisk drive, the operation of which will be briefly described withreference to FIGS. 1-3. FIG. 1 is a block diagram of a conventionalmagnetic recording hard disk drive. The disk drive includes a magneticrecording disk 12 and a rotary voice coil motor (VCM) actuator 14supported on a disk drive housing or base 16. The disk 12 has a centerof rotation 13 and is rotated in direction 15 by a spindle motor (notshown) mounted to base 16. The actuator 14 pivots about axis 17 andincludes a rigid actuator arm 18. A generally flexible suspension 20includes a flexure element 23 and is attached to the end of arm 18. Ahead carrier or air-bearing slider 22 is attached to the flexure 23. Amagnetic recording read/write head 24 is formed on the trailing surface25 of slider 22. The flexure 23 and suspension 20 enable the slider to“pitch” and “roll” on an air-bearing generated by the rotating disk 12.Typically, there are multiple disks stacked on a hub that is rotated bythe spindle motor, with a separate slider and read/write head associatedwith each disk surface.

FIG. 2 is an enlarged end view of the slider 22 and a section of thedisk 12 taken in the direction 2-2 in FIG. 1. The slider 22 is attachedto flexure 23 and has an air-bearing surface (ABS) 27 facing the disk 12and a trailing surface 25 generally perpendicular to the ABS. The ABS 27causes the airflow from the rotating disk 12 to generate a bearing ofair that supports the slider 22 in very close proximity to or nearcontact with the surface of disk 12. The read/write head 24 is formed onthe trailing surface 25 and is connected to the disk drive read/writeelectronics by electrical connection to terminal pads 29 on the trailingsurface 25.

FIG. 3 is a view in the direction 3-3 of FIG. 2 and shows the ends ofread/write head 24 as viewed from the disk 12. The read/write head 24 isa series of thin films deposited and lithographically patterned on thetrailing surface 25 of slider 22. The write head includes magnetic writepoles P1/S2 and P1 separated by a write gap 30. The CPP-SVmagnetoresistive sensor or read head 100 is located between two magneticshields S1 and P1/S2, with P1/S2 also serving as the first write polefor the write head. The shields S1, S2 are formed of magneticallypermeable material and are electrically conductive so they can functionas the electrical leads to the read head 100. Separate electrical leadsmay also be used, in which case the read head 100 is formed in contactwith layers of electrically conducting lead material, such as tantalum,gold, or copper, that are in contact with the shields S1, S2.

FIG. 4 is an enlarged sectional view showing the layers making up sensor100 as seen from the air bearing surface (ABS) of the sensor. Sensor 100is a CPP-SV read head comprising a stack of layers formed between thetwo magnetic shield layers S1, S2 that are typically electroplated NiFealloy films. The lower shield S1 is typically polished bychemical-mechanical polishing (CMP) to provide a smooth substrate forthe growth of the sensor stack. This may leave an oxide coating whichcan be removed with a mild etch just prior to sensor deposition. Thesensor layers include a pinned ferromagnetic layer 120 having a fixedmagnetic moment or magnetization direction 121 oriented substantiallyperpendicular to the ABS (into the page), a free ferromagnetic layer 110having a magnetic moment or magnetization direction 111 that is orientedsubstantially parallel to the ABS in the absence of an external magneticfield and that can rotate in the plane of layer 110 in response totransverse external magnetic fields from the disk 12, and anelectrically conducting spacer layer 130, typically copper (Cu), betweenthe pinned layer 120 and the free layer 110.

The pinned ferromagnetic layer in a CPP-SV sensor may be a single pinnedlayer or an antiparallel (AP) pinned structure. An AP-pinned structurehas first (AP1) and second (AP2) ferromagnetic layers separated by anonmagnetic antiparallel coupling (APC) layer with the magnetizationdirections of the two AP-pinned ferromagnetic layers orientedsubstantially antiparallel. The AP2 layer, which is in contact with thenonmagnetic APC layer on one side and the sensor's electricallyconducting spacer layer on the other side, is typically referred to asthe reference layer. The AP1 layer, which is typically in contact withan antiferromagnetic or hard magnet pinning layer on one side and thenonmagnetic APC layer on the other side, is typically referred to as thepinned layer. The AP-pinned structure minimizes the net magnetostaticcoupling between the reference/pinned layers and the CPP-SV freeferromagnetic layer. The AP-pinned structure, also called a “laminated”pinned layer, and sometimes called a synthetic antiferromagnet (SAF), isdescribed in U.S. Pat. No. 5,465,185.

The pinned layer in the CPP-SV sensor in FIG. 4 is an AP-pinnedstructure with reference ferromagnetic layer 120 (AP2) and a lowerferromagnetic layer 122 (AP1) that are antiferromagnetically coupledacross an AP coupling (APC) layer 123. The APC layer 123 is typicallyRu, Ir, Rh, Cr or alloys thereof. The free ferromagnetic layer 110,spacer layer 130 and AP2 layer 120 together make up what is call the“active region” of the sensor. The AP1 and AP2 ferromagnetic layers havetheir respective magnetization directions 127, 121 orientedantiparallel. The API layer 122 may have its magnetization directionpinned by being exchange-coupled to an antiferromagnetic (AF) layer 124as shown in FIG. 4. Alternatively, the AP-pinned structure may be“self-pinned” or it may be pinned by a hard magnetic layer such asCo_(100-x)Pt_(x) or Co_(100-x-y)Pt_(x)Cr_(y) (where x is about between 8and 30 atomic percent). In a “self pinned” sensor the AP1 and AP2 layermagnetization directions 127, 121 are typically set generallyperpendicular to the disk surface by magnetostriction and the residualstress that exists within the fabricated sensor. It is desirable thatthe AP1 and AP2 layers have similar moments. This assures that the netmagnetic moment of the AP-pinned structure is small so thatmagneto-static coupling to the free layer is minimized and the effectivepinning field of the AF layer 124, which is approximately inverselyproportional to the net magnetization of the AP-pinned structure,remains high. In the case of a hard magnet pinning layer, the hardmagnet pinning layer moment needs to be accounted for when balancing themoments of AP1 and AP2 to minimize magnetostatic coupling to the freelayer.

Located between the lower shield layer S1 and the AP-pinned structureare the bottom electrical lead 126 and a seed layer 125. The seed layer125 may be a single layer or multiple layers of different materials.Located between the free ferromagnetic layer 110 and the upper shieldlayer S2 are a capping layer 112 and the top electrical lead 113. Thecapping layer 112 may be a single layer or multiple layers of differentmaterials, such as a Cu/Ru/Ta trilayer.

In the presence of an external magnetic field in the range of interest,i.e., magnetic fields from recorded data on the disk 12, themagnetization direction 111 of free layer 110 will rotate while themagnetization direction 121 of reference layer 120 will remainsubstantially fixed and not rotate. The rotation of the free-layermagnetization 111 relative to the reference-layer magnetization 121results in a change in electrical resistance. Hence, when a sensecurrent I_(S) is applied between top lead 113 and bottom lead 126, theresistance change is detected as a voltage signal proportional to thestrength of the magnetic signal fields from the recorded data on thedisk.

The leads 126, 113 are typically Ta or Rh. However, any low resistancematerial may also be used. They are optional and used to adjust theshield-to-shield spacing. If the leads 126 and 113 are not present, thebottom and top shields S1 and S2 are used as leads. The seed layer 125is typically one or more layers of NiFeCr, NiFe, Ta, Cu or Ru. The AFlayer 124 is typically a Mn alloy, e.g., PtMn, NiMn, FeMn, IrMn, IrMnCr,PdMn, PtPdMn or RhMn. If a hard magnetic layer is used instead of an AFlayer it is typically a CoPt or FePt alloy, for example CoPtCr. Thecapping layer 112 provides corrosion protection and is typically formedof Ru or Ta.

The ferromagnetic layers 122 (AP1), 120 (AP2), and 110 (free layer) aretypically formed of crystalline CoFe or NiFe alloys, or a multilayer ofthese materials, such as a CoFe/NiFe bilayer. These alloys have asufficiently high magnetic moment M and bulk electron scatteringparameter β, but a relatively low electrical resistivity ρ. The AP2layer can also be a laminated structure to obtain a high degree ofspin-dependent interface scattering. For example the AP2 layer can be aFM/XX/FM/ . . . /XX/FM laminate, where the ferromagnetic (FM) layers areformed of Co, Fe or Ni, one of their alloys, or a multilayer of thesematerials, such as a CoFe—NiFe—CoFe trilayer; and the XX layers arenonmagnetic layers, typically Cu, Ag, or Au or their alloys, and arethin enough that the adjacent FM layers are strongly ferromagneticallycoupled.

For example, AP2 layer 120 may be a CoFe alloy, typically 10 to 30 Åthick, and the free ferromagnetic layer 110 may be a bilayer of a CoFealloy, typically 10-15 Å thick and formed on the spacer layer 130, witha NiFe alloy, typically 10-30 Å thick, formed on the CoFe layer of thebilayer. The APC layer in the AP-pinned structure is typically Ru or Irwith a thickness between about 4-10 Å.

If the AP-pinned structure is the “self-pinned” type, then noantiferromagnetic pinning layer is required. In a self-pinned structurewhere no antiferromagnet or hard magnet pinning layer is present, theAP1 layer is in contact with a seed layer on the sensor substrate.

A ferromagnetic biasing layer 115, such as a CoPt or CoCrPt hardmagnetic bias layer, is also typically formed outside of the sensorstack near the side edges of the free ferromagnetic layer 110. Thebiasing layer 115 is electrically insulated from free layer 110 byinsulating regions 116, which may be formed of alumina, for example. Thebiasing layer 115 has a magnetization 117 generally parallel to the ABSand thus longitudinally biases the magnetization 111 of the free layer110. Hence in the absence of an external magnetic field itsmagnetization 117 is parallel to the magnetization 111 of the free layer110. The ferromagnetic biasing layer 115 may be a hard magnetic biaslayer or a ferromagnetic layer that is exchange-coupled to anantiferromagnetic layer.

One or more of the free layer 110, the AP2 layer 120, the capping layer112 and the conductive nonmagnetic spacer layer 130 may also include anano-oxide layer (NOL) to locally confine the current path and increasethe effective resistance of the active region. A CoFe NOL may be formed,for example, by interrupting the deposition after some CoFe has beendeposited somewhere in the free layer, the AP2 layer, the capping layer,or the conductive spacer layer and oxidizing its surface for severalminutes in an O₂ or O₂/Ar gas at 0.1-10 Torr. NOLs can be formed byoxidizing other materials, e.g., Cu/Al or Cu/Ti alloys or multilayers.

While the read head 100 shown in FIG. 4 is a “bottom-pinned” read headbecause the AP-pinned structure is below the free layer 110, the freelayer 110 can be located below the AP-pinned structure. In such anarrangement the layers of the AP-pinned structure are reversed, with theAP2 layer 120 on top of and in contact with the spacer layer 130.

In this invention the CPP sensor is similar to the CPP sensor describedabove, but has an antiparallel-free (APF) structure as the free layer,and a specific direction for the applied bias or sense current, thatresult in both increased magnetoresistance and reduced susceptibility tocurrent-induced instability and noise. While the invention will bedescribed with respect to a CPP-SV read head, the invention isapplicable to other types of CPP magnetoresistive sensors such as MTJsensors, and to CPP sensors with NOLs.

The CPP-SV sensor of this invention is illustrated in FIG. 5 as sensor200. The free layer is an antiparallel free (APF) structure comprising afirst free ferromagnetic layer 201 (FL1), second free ferromagneticlayer 202 (FL2), and an antiparallel (AP) coupling (APC) layer 203. APClayer 203, such as a thin (between about 4 Å and 10 Å) Ru film, couplesFL1 and FL2 together antiferromagnetically with the result that FL1 andFL2 maintain substantially antiparallel magnetization directions, asshown by arrows 211, 212, respectively. Theantiferromagnetically-coupled FL1 and FL2 rotate together in thepresence of a magnetic field, such as the magnetic fields from datarecorded in a magnetic recording medium. The net magnetic moment/area ofthe APF structure (represented by the difference in magnitudes of arrows211, 212) is (M1*t1−M2*t2), where M1 and t1 are the saturationmagnetization and thickness, respectively, of FL1, and M2 and t2 are thesaturation magnetization and thickness, respectively, of FL2. Thus thethicknesses of FL1 and FL2 are chosen to obtain the desired net freelayer magnetic moment for the sensor. In advanced CPP read heads, it isdesirable that the net magnetic moment/area of the free layer(M1*t1−M2*t2), be no greater than the equivalent moment/area ofapproximately 100 Å of NiFe. As used herein in reference to equivalentmoments and equivalent thicknesses, NiFe shall mean a Ni₈₀Fe₂₀ alloy,i.e. a NiFe alloy with 20 atomic percent Fe.

In FIG. 5, the free ferromagnetic layer 201 (FL1) is shown as having thegreater magnetic moment, as represented by the greater length of arrow211. Thus a ferromagnetic biasing layer 215, which may be a hardmagnetic layer such as a CoPt or CoCrPt layer, is formed outside of thesensor stack near the side edges of the free ferromagnetic layer 201(FL1) and has a magnetization 217 that is parallel to the ABS. Thebiasing layer 215 is electrically insulated from FL1 by insulatingregions 216, which may be formed of alumina, for example. Thus biasinglayer 215 longitudinally biases the net magnetization of the APFstructure and its magnetization 217 is parallel to the magnetization 211of FL1 in the absence of an external magnetic field. However, as shownin FIG. 5, the magnetization 217 of biasing layer 215 is antiparallel tothe magnetization 212 of FL2 layer 202 in the absence of an externalmagnetic field. As a result, if the magnetic moment of FL2 is too high,which can occur if FL2 is too thick, the APF structure may bemagnetostatically unstable. Thus while the thicknesses of FL1 and FL2can be chosen to obtain a desired net free layer magnetic moment for thesensor, it is at the same time desirable to keep FL2 as thin aspossible, not only for magnetostatic stability, but to also minimize theoverall thickness of the sensor stack.

An APF structure as a free layer in a CIP sensor was first described inU.S. Pat. No. 5,408,377. A specific type of Fe-based APF structure as afree layer in a CPP sensor is described in US 2005/0243477 A1, assignedto the same as assignee as this application. In the sensor described inthat application the objective is high magnetoresistance, so the freelayer material near the spacer layer is Fe and the spacer layer is thusrequired to be Cr because Fe/Cr multilayers are a well-known systemexhibiting giant magnetoresistance. The use of pure Fe in the free layernear the spacer layer requires that a second free layer farther from thespacer layer be used to lower the coercivity of the Fe-based APF andthat the second free layer be alloyed with nitrogen (N) to furtherreduce the coercivity. However, it is not preferred to use pure elementssuch as Fe in the free layer or any other active magnetic layer of a CPPsensor. Rather, it is preferred to use an alloy, such as a FeCo alloy,to increase electron scattering and thus to shorten the spin-diffusionlength. This is important to achieve high magnetoresistance with thinmagnetic layers, resulting in a thin sensor capable of high resolution.Also, pure Fe is corrosive and should be avoided for reliabilityreasons.

In this invention the spacer layer 230 is not formed of Cr, but ispreferably Cu, Au or Ag which yield higher CPP magnetoresistance incontact with Co_(x)Fe_(1-x) (50<x<100 at. %) and Ni_(x)Fe_(1-x)(50<x<100 at. %) magnetic layers. Also, in this invention FL1 and FL2are not formed of Fe or FeN due to their susceptibility to corrosion atthe air-bearing surface, but may be formed of crystalline CoFe or NiFealloy, or a multilayer of these materials, such as a CoFe/NiFe bilayer.FL1 and FL2 may also be formed of relatively high-resistance amorphousalloys, such as an alloy of one or more elements selected from Co, Feand Ni, and at least one nonmagnetic element that is present in anamount that renders the otherwise crystalline alloy amorphous. Examplesof amorphous alloys for FL1 and FL2 include Co_((100-x-y))Fe_(x)X_(y)and Ni_((100-x-y))Fe_(x)X_(y) where X is B, Si or Tb, and y is betweenabout 5 and 40 atomic percent (at .%). FL1 and FL2 may also be formed ofa ferromagnetic Heusler alloy, i.e., a metallic compound having aHeusler alloy crystal structure. Examples of Heusler alloys for FL1 andFL2 include Co₂MnX (where X is Al, Sb, Si or Ge), NiMnSb, PtMnSb, andCo₂Fe_(x)Cr_((1-x))Al (where x is between 0 and 1). FL1 and FL2 may alsobe formed of a high-resistance ferromagnetic crystalline alloy based onCoFe or NiFe alloys, with short spin diffusion length (<100 Å) andresulting in appreciable CPP magnetoresistance, and containing one ormore additions of Cu, Mg, Al, Si, Au, Ag or B. Also, in this inventionthe APC layer 203 is not formed of Cr, but is preferably Ru, Ir, or Rhor alloys thereof.

In this invention, it can be advantageous to select the composition andthickness of FL1 to be greater than the spin-diffusion length of theelectrons in the FL1 material to maximize the bulk spin-dependentscattering of electrons and thus maximize the sensor signal. If the FL1thickness is significantly less than the spin-diffusion length, (whichis about 40 Å for NiFe), then the signal due to FL1 is not maximized. Inaddition, FL2 may participate in the spin-valve effect and may, due toits magnetization being opposite in direction to that of FL1, cause anegative contribution to the magnetoresistance. If FL1 is sufficientlythick (substantially above the spin-diffusion length or at least about110% of the spin-diffusion length), then the signal from FL1 is close tomaximum, and there is no appreciable negative spin-valve contributionfrom FL2. The spin-diffusion length depends on the magnetic material andis about 40 Å for NiFe, 500 Å for Co, and 120 Å for Co₉₀Fe₁₀. Ifmaterials that have a high spin-diffusion length are used for FL1 thenthe APF structure can be made thicker if an optimum signal amplitude isdesired, but that will increase the overall shield-to-shield spacing ofthe sensor. The spin-diffusion length in any material may be determinedthrough a series of experiments measuring the dependence of the CPPspin-valve effect on the thickness of the material of interest. Thematerial of interest may be included either in the spacer layer or besubstituted for one of the ferromagnetic layers. See for example A. C.Reilly et al., J. Mag. Mag. Mat. 195, L269-L274 (1999). Theseexperiments are somewhat difficult and tedious, so the spin-diffusionlength is not yet known for a large number of materials. One generaltrend is that the spin-diffusion length varies inversely with theelectrical resistivity, i.e., materials with a large electricalresistivity display a short spin-diffusion length. In general, alloysexhibit higher resistivity than pure metals, i.e, unalloyed elementalmetals, and thus shorter spin-diffusion lengths. Thus, in general,alloys tend to have shorter spin diffusion lengths than pure metalsbecause they exhibit a larger resistivity than the elements they arecomprised of due to enhanced electron scattering.

In this invention, as shown in FIG. 5, the direction of the sensecurrent I_(S) (which is the conventional current and opposite indirection to the electron current) from current source 150 isspecifically chosen to be from the AP-pinned structure to the APFstructure (from bottom lead 126 to top lead 113), so that possiblespin-torque induced instability is suppressed, as will be explainedbelow. To achieve this reduction in spin-torque instability, orconcomitantly, increase in the critical current for instability onset,it has been determined that FL2 must be sufficiently thick. At the sametime, as explained above, if FL2 is too thick the APF structure may bemagnetostatically unstable as a result of the magnetization of F2 beingantiparallel to the magnetization of the ferromagnetic biasing layer.Thus, in this invention the thickness of FL2 is chosen to be as thin aspossible, but thick enough to assure that the spin-torque inducedinstability of the free-layer (or APF structure) is not the limitingfactor in the performance of the read head. From experimental datadescribed below FL2 can be as thin as approximately 10 Å of Ni₈₀Fe₂₀equivalent moment, and still provide sufficient immunity fromspin-torque instability.

CPP test samples with APF structures of this invention (similar to FIG.5) were compared with a CPP control sample with a single free layer(similar to FIG. 4). The material for the free layers was a NiFe alloywith 2 at. % Tb (Ni₈₃Fe₁₅Tb₂), where the subscripts represent atomicpercent. The control had a free layer thickness of 40 Å, a ΔRA of about0.75 mOhm-μm², and a ΔR/R of about 2.35%. In the CPP test sample if theAPF structure had FL1=50 Å and FL2=10 Å there was no significantdifference in ΔRA from the control. This is likely because the positiveeffect of the thicker FL1 (50 Å compared to the 40 Å control) iscompensated by the negative effect of the FL2 layer. However, if FL2 isthicker (FL2=20 Å and FL1=60 Å; or FL2=30 Å and FL1=70 Å), then the ΔRAsignal is higher than the control (0.87 mOhm-μm² and 0.90 mOhm-μm² forFL1=60 Å and 70 Å, respectively). Although FL2 increased in thickness,it did not result in any noticeable decrease in ΔRA because FL1 issubstantially greater than the spin-diffusion length in NiFe or NiFeTb(which is at most about 40-50 Å), and the Ru APC layer (layer 203 inFIG. 5) further mixes the electron spins, so that FL2 becomes“disconnected” from the FL1 reference layer (layer 120 in FIG. 5) andcannot significantly participate in the spin-valve effect. For the testsample with FL1=50 Å and FL2=10 Å the ΔR/R decreased from the controlvalue of 2.35% to 2.15%, but for the test sample with FL1=60 Å andFL2=20 Å the ΔR/R then increased to about 2.5%. As FL1 is made eventhicker (keeping the FL2−FL1 thickness difference at about 40 Å) it isexpected from the considerations above that ΔRA would saturate to anearly constant value, while ΔR/R would also saturate but eventuallydecrease as RA increases with additional layer thickness. Thus CPPsensors with thicker FL1 layers (FL1=60 Å or FL1=70 Å) are optimized fora magnetoresistive response.

In another set of test samples, the APF layer structure comprised atrilayer FL1=CoFe(6 Å)/NiFe(t1)/CoFe(2 Å) and a bilayer FL2=CoFe(2Å)/NiFe(t2), and the thicknesses t1 and t2 were chosen so that the netmoment/area (M1*t1−M2*t2) of the APF structure was constant at theequivalent moment/area of about 45 Å of NiFe. The control sample with asimple free layer (no FL2, and FL1=6 Å CoFe/38 Å NiFe/2 Å CoFe) had aΔR/R of about 2.13%. For the test samples with an APF structure and t2values from 5 Å to 45 Å (t1 values from 45 Å to 85 Å) the ΔR/R increasedto about 2.7% to 2.8%. The ΔRA increased from 0.68 mOhm-μm² for thesample with the simple free layer structure to 0.93 mOhm-μm² for thesample with the APF structure with t2=15 Å of NiFe.

An important feature of this invention, in addition to an improvedmagnetoresistance, is that the CPP sensor exhibits substantial immunityto current-induced noise when the bias or sense current I_(S) is in thedirection from the pinned layer to the APF structure (from lead 126 tolead 113 in FIG. 5). For the purpose of FIGS. 6-7, this actual sensecurrent direction corresponds to “negative” electron current I_(e)(i.e., the electrons flow from the APF structure to the pinned layer).

FIG. 6 shows the noise power spectral density (PSD) for a control samplewith a single free layer (FL2=none; t1=38 Å NiFe). Positive appliedmagnetic fields (curve 300) correspond to the pinned and free layer netmagnetizations being antiparallel, and negative applied fields (curve310) correspond to the pinned and free layer net magnetizations beingparallel. It can be seen that for a “positive” electron current I_(e)(actual sense current I_(S) from the free layer to the pinned layer orfrom top to bottom in FIG. 4) the antiparallel curve 300 shows a largeincrease in noise power above a “critical current” of about 0.6 mA,while for negative electron currents the noise increase for the parallelorientation (curve 310) is above critical current of about 1.5 mA. Thisbehavior is expected in the present understanding of current-inducednoise in spin-valve structures. By comparison with FIG. 6, FIG. 7 showsthe PSD for a sample with an APF structure like this invention, witht1=65 Å NiFe and t2=25 Å NiFe. Positive applied magnetic fields (curve400) correspond to the pinned and free layer net magnetizations beingantiparallel, and negative applied fields (curve 410) correspond to thepinned and free layer net magnetizations being parallel.

A comparison of FIGS. 6 and 7 shows that while the behavior for positiveelectron currents is qualitatively similar (i.e., a large increase innoise power above a critical current of about 0.6 mA), the behavior fornegative electron currents is unexpected and unique. In particular, inFIG. 7 the noise power remains extremely low for both parallel andantiparallel orientations, up to large values of the electron current(“more negative” than a critical current of about 6.5 mA, although theexact value is not known due to limitations in the measurement method).

FIG. 8 is a graph of positive and negative critical electron currentsfor spin-torque-induced noise as a function of the FL2 thickness.(Negative electron current corresponds to electrons flowing from the APFstructure to the pinned layer, which corresponds to bias or sensecurrent from the pinned layer to the APF structure). As shown in FIG. 8,the negative critical current increases (becomes more negative) withincreasing FL2 thickness. Sensors with FL2 equal to and greater thanabout 15 Å NiFe will exhibit high negative critical currents forspin-torque instability. Thus, there is a range of APF structures (withappropriate FL1 and FL2 equivalent NiFe thicknesses) which display thisspin-torque immunity for negative electron current (bias or sensecurrent I_(S) in the direction from the pinned layer to the APFstructure), in addition to an improved magnetoresistive response.

Thus FIG. 8 shows that FL2 must have some minimum thickness tocontribute to the sensor's immunity to spin-torque instability fornegative electron current. However, as described above, it is alsodesirable to keep FL2 as thin as possible to assure magnetostaticstability of the APF structure as well as to minimize the overallthickness of the sensor stack.

FIG. 9 is a numerical simulation of the negative critical currentdensity (for the parallel state of APF structure and pinned layers) foran APF structure, as a function of the FL1 thickness (equivalent NiFethickness) for four values (0, 5 Å, 10 Å, and 15 Å) of FL2 thickness.The values are similar to that described in N. Smith et al., “CoresonantEnhancement of Spin-Torque Critical Currents in Spin Valves with aSynthetic-Ferrimagnet Free Layer”, PHYSICAL REVIEW LETTERS, 101, 247205(2008). FIG. 9 demonstrates that the fractional (or percentage) increasein negative critical current relative to the FL2=0 control case isgreater for larger values of FL2/FL1. However, more importantly, it alsodemonstrates that maintaining an absolute value of FL2=15 Å NiFe issufficient to give a substantial, i.e., greater than three-fold,increase in negative critical current (relative to the FL2=0 control)even for FL1 thicknesses up to 100 Å NiFe, and that the absolute valueof the negative critical current for the APF structure is relativelyindependent of FL1 thickness when FL2 is fixed at a thickness resultingin a magnetic moment equivalent to 15 Å NiFe. Practical experience hasshown that a three-fold increase in critical current is more thansufficient margin such that spin-torque-induced instability of thefree-layer is not the limiting factor on read head performance. Thus,although the critical current for the APF structure would be predictedto quickly increase even further with thicker FL2, such a thicker FL2will offer little to no spin-torque stability advantage for the readhead, but will quickly become costly in weakening magnetostaticstability.

While an FL2 with a thickness no greater than that resulting in amagnetic moment equivalent to 15 Å NiFe provides a sufficient criticalcurrent and is thus the preferred or optimum minimum thickness, FIG. 9shows that a FL2 with a thickness no greater than that resulting in amagnetic moment equivalent to 10 Å NiFe will still result in asignificant increase in negative critical current. Thus in the presentinvention the minimum thickness for FL2 is a thickness resulting in aFL2 magnetic moment equivalent to approximately 10 Å NiFe and preferablyto approximately 15 Å NiFe. Thus a CPP sensor with an APF structure andsense current applied in a direction from the pinned layer to the APFstructure can be achieved if FL2 has a thickness equal to or greaterthan a thickness that results in a FL2 magnetic moment equivalent to 10Å Ni₈₀Fe₂₀ and equal to or less than a thickness that results in a FL2magnetic moment equivalent to 15 Å Ni₈₀Fe₂₀.

Thus the CPP sensor according to this invention allows a much largerbias or sense current to be applied before current-induced noise occurs,provided the sense current I_(S) is applied in the direction from thepinned layer to the APF structure (electron current I_(e) from the APFstructure to the pinned layer). The increase in critical current forcurrent-induced noise by a factor of three or more can provide acorresponding increase in output voltage for the sensor.

In this invention the effect on sensor operation due to the increase incritical current (by a factor of three or more) is potentially muchlarger than the increase in output voltage due to the increase inmagnetoresistance (by 10-30%) from the APF structure. Consequently, ifthe thinnest possible sensor (or a much smaller free layermagnetization) is desired, the increase in critical current, andcorresponding increase in sensor output voltage, can still be realizedusing thinner FL1 and FL2 layers in the APF structure (andcorrespondingly smaller magnetoresistance).

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A magnetoresistive sensor having electrical leads for connection to asource of sense current and comprising: one and only one pinnedferromagnetic layer having an in-plane magnetization direction; anantiparallel free (APF) structure comprising a first free ferromagneticlayer (FL1) having an in-plane magnetization, a second freeferromagnetic layer (FL2) having an in-plane magnetization substantiallyantiparallel to the magnetization of FL1 and a magnetic moment less thanthat of FL1, and an AP coupling (APC) layer between and in contact withFL1 and FL2 and consisting essentially of a material selected from thegroup consisting of Ru, Ir, Rh and alloys thereof, wherein FL2 has athickness equal to or greater than a thickness that results in a FL2magnetic moment equivalent to 10 Å Ni₈₀Fe₂₀; and an electricallyconductive spacer layer between the pinned ferromagnetic layer and FL1and consisting essentially of an element selected from the groupconsisting of Cu, Ag and Au; and wherein the sensor is capable ofsensing external magnetic fields when a sense current is appliedperpendicular to the planes of the layers in the sensor in a directionfrom the pinned ferromagnetic layer to the APF structure.
 2. The sensorof claim 1 wherein FL2 has a thickness equal to or greater than athickness that results in a FL2 magnetic moment equivalent to 15 ÅNi₈₀Fe₂₀.
 3. The sensor of claim 1 wherein FL2 has a thickness equal toor greater than a thickness that results in a FL2 magnetic momentequivalent to 10 Å Ni₈₀Fe₂₀ and equal to or less than a thickness thatresults in a FL2 magnetic moment equivalent to 15 Å Ni₈₀Fe₂₀.
 4. Thesensor of claim 1 further comprising a ferromagnetic biasing layer forbiasing the magnetization of the APF structure, the biasing layer havinga magnetization substantially antiparallel to the magnetization of FL2in the absence of an external magnetic field.
 5. The sensor of claim 4wherein the biasing layer is a hard magnetic layer.
 6. The sensor ofclaim 1 further comprising a substrate, a first electrical lead layer onthe substrate, and a second electrical lead layer.
 7. The sensor ofclaim 6 wherein the pinned ferromagnetic layer is on the first leadlayer, the spacer layer is on the pinned ferromagnetic layer, FL1 is onthe spacer layer, the second lead layer is on the APF structure, and thedirection of sense current to be applied to the sensor is in thedirection from the first lead layer to the second lead layer.
 8. Thesensor of claim 1 wherein FL1 comprises a crystalline alloy of Fe and atleast one of Ni and Co.
 9. The sensor of claim 1 wherein FL1 comprisesan amorphous ferromagnetic alloy comprising a nonmagnetic element X andat least one of Co, Ni and Fe.
 10. The sensor of claim 1 wherein FL1comprises a ferromagnetic Heusler alloy.
 11. The sensor of claim 1wherein the net magnetic moment/area of the APF structure is less thanthe magnetic moment/area of 100 Å of Ni₈₀Fe₂₀.
 12. The sensor of claim 1wherein the thickness of FL1 is at least 110% of the spin diffusionlength of the material from which FL1 is formed.
 13. The sensor of claim1 wherein FL1 is formed essentially of Ni₈₀Fe₂₀ and the thickness of FL1is at least 45 Å.
 14. The sensor of claim 1 wherein the current-inducednoise of the sensor when the sense current is applied perpendicular tothe planes of the layers in the sensor in a direction from the pinnedferromagnetic layer to the APF structure is substantially less than thecurrent-induced noise when sense current is in a direction from the APFstructure to the pinned ferromagnetic layer.
 15. The sensor of claim 1wherein the pinned ferromagnetic layer comprises an antiparallel (AP)pinned structure comprising a first AP-pinned (AP1) ferromagnetic layerhaving an in-plane magnetization direction, a second AP-pinned (AP2)ferromagnetic layer having an in-plane magnetization directionsubstantially antiparallel to the magnetization direction of the AP1layer, and an AP coupling (APC) layer between and in contact with theAP1 and AP2 layers; and wherein the electrically conductive spacer layeris on the AP2 layer.
 16. The sensor of claim 15 wherein the AP-pinnedstructure is a self-pinned structure.
 17. The sensor of claim 15 furthercomprising an antiferromagnetic layer exchange-coupled to the AP1 layerfor pinning the magnetization direction of the AP1 layer.
 18. The sensorof claim 15 further comprising a hard magnetic layer in contact with theAP1 layer for pinning the magnetization direction of the AP1 layer. 19.The sensor of claim 1 wherein the sensor is a magnetoresistive read headfor reading magnetically recorded data from tracks on a magneticrecording medium, and wherein the electrical leads are shields formed ofmagnetically permeable material.
 20. Acurrent-perpendicular-to-the-plane magnetoresistive read head forreading magnetically recorded data from tracks on a magnetic recordingmedium when a sense current is applied to the head in a direction from afirst electrical lead to a second electrical lead, the head comprising:a substrate; a first electrical lead layer on the substrate; one andonly one pinned ferromagnetic layer, said one and only one pinnedferromagnetic layer being located on the first electrical lead layer andhaving an in-plane magnetization direction; an electrically conductivespacer layer on the pinned ferromagnetic layer and consistingessentially of an element selected from the group consisting of Cu, Agand Au; an antiparallel free (APF) structure comprising (a) a first freeferromagnetic layer (FL1) on the spacer layer and having an in-planemagnetization direction, (b) a second free ferromagnetic layer (FL2)having an in-plane magnetization direction substantially antiparallel tothe magnetization direction of FL1 and a magnetic moment less than thatof FL1, wherein each of FL1 and FL2 is formed of an alloy comprising Feand one or both of Ni and Co and FL2 has a thickness equal to or greaterthan a thickness that results in a FL2 magnetic moment equivalent to 10Å Ni₈₀Fe₂₀, and (c) an AP coupling (APC) layer between and in contactwith FL1 and FL2 and consisting essentially of a material selected fromthe group consisting of Ru, Ir, Rh and alloys thereof; a secondelectrical lead layer on FL2; a ferromagnetic biasing layer for biasingthe magnetization of the APF structure, the biasing layer having amagnetization direction substantially antiparallel to the magnetizationdirection of FL2 in the absence of an external magnetic field; and asource of sense current connected to the first and second electricallead layers and supplying current to the sensor in a direction from thefirst electrical lead layer through said one and only one pinnedferromagnetic layer, spacer layer, and APF structure to the secondelectrical lead layer.
 21. The head of claim 20 wherein FL2 has athickness equal to or greater than a thickness that results in a FL2magnetic moment equivalent to 15 Å Ni₈₀Fe₂₀.
 22. The head of claim 20wherein FL2 has a thickness equal to or greater than a thickness thatresults in a FL2 magnetic moment equivalent to 10 Å Ni₈₀Fe₂₀ and equalto or less than a thickness that results in a FL2 magnetic momentequivalent to 15 Å Ni₈₀Fe₂₀.
 23. The head of claim 20 wherein thebiasing layer is a hard magnetic layer located near the edges of FL1.24. The head of claim 20 wherein the net magnetic moment/area of the APFstructure is less than the magnetic moment/area of 100 Å of Ni₈₀Fe₂₀.25. The head of claim 20 wherein the thickness of FL1 is at least 110%of the spin diffusion length of the material from which FL1 is formed.26. The head of claim 20 wherein the current-induced noise of the headwhen sense current is applied in a direction from said first electricallead layer to said second electrical lead layer is substantially lessthan the current-induced noise when sense current is applied in adirection from said second electrical lead layer to said firstelectrical lead layer.
 27. The head of claim 20 wherein the pinnedferromagnetic layer comprises an antiparallel (AP) pinned structurecomprising a first AP-pinned (AP1) ferromagnetic layer having anin-plane magnetization direction, a second AP-pinned (AP2) ferromagneticlayer having an in-plane magnetization direction substantiallyantiparallel to the magnetization direction of the AP1 layer, and an APcoupling (APC) layer between and in contact with the AP1 and AP2 layers;and wherein the electrically conductive spacer layer is on the AP2layer.